Download linked genes

LINKED GENES
Genes are found on chromosomes, right? So if some genes are on the same chromosome why aren’t they
inherited all linked together as a package deal on the same chromosome? (But hey, that would be a
contradiction of Mendel’s law of Independent Assortment, would it not?!)
As a matter of fact – some genes are linked in this manner. William Bateson was the
famous scientist who “rediscovered” Mendel, who invented the term “Genetics” and
was the first to recognize that some genes are linked.
Numerous genes are found on the same chromosome.
Genes on same chromosome are described as being in same Linkage Groups and
inherited together. For example, red hair and freckles are closely linked; you rarely
get one without the other.
Such genes are unable to assort independently- eg. will not show F1 self-cross 9:3:3:1
ratio.
Let’s assume mugwumps possess two genes: Attitude vs shyness (A vs a) and Big vs. Small Feet (B vs. b).
Scenario 1 - Let’s assume NO linkage:
a dihybrid male is test-crossed with a homozygous recessive shy small foot.
ab
AB
Ab
aB
ab
AaBb
Aabb
aaBb
aabb
This is what Mendel would expect according to the law of independent assortment.
Scenario 2 - In fact, these genes are closely linked on same chromosome – assume complete linkage:
A purebred attitudinal large footed male crosses with a homozygous recessive shy small foot.
The F1 all are
___A____B__
notice the allele linkages from parents
____________
a
b
An F1 male is test-crossed with a homozygous recessive female. Notice that the male can only produce two
kinds of sperm – not four as before when there was no linkage:
___A____B__ ___a____b__
___A____B__ ___a____b__
___________ ___________
___a____b__
a
b
a
b
This is not what Mendel would expect - independent assortment failed to occur because of linkage.
Tom Mueller RHS
Scenario 3 – In fact such a Mugwump test cross produces offspring with the following ratio
47.5 %
47.5 %
2.5%
2.5%
attitudinal, large footed
shy, small footed
attitudinal, small footed
shy, large footed
How is this possible? Clearly most of the time – the linked genes stay linked. But rarely (5% of the time) some
independent assortment does take place.
___A____B__ ___a____b__ ___A____b__ ___a____B__
(47.5%)
(47.5%)
(2.5%)
(2.5%)
___A___B__ ___a____b__ ___A____b__ ___a____B__
___________ ___________ ___________ ___________ ___________
a
b
a
b
a
b
a
b
a
b
Remember – these insights were taking place before anybody understood DNA and
chromosomes.
Morgan (working with fruit flies) was the first to demonstrate that genes are carried
on chromosomes and are the mechanical basis of heredity.
Imagine crossing over or swapping of DNA during the production of “sex cells” or
gametes:
Or in the case of our Mugwumps:
____A__________________________________________B__________________
____A___________
__________________________B__________________
____a____________
__________________________ b__________________
____a___________ _______________________________b__________________
What would be the outcome of such cross-over events?
___A____B__
and ___a____b__
But on rare occasion – we also see
___A____b__ and ___a____B__
most of the time
some of the time - Recombinant chromosomes!
The closer together the genes – the less likely we will see such a cross-over during the test-cross.
The further apart the genes – the more likely we will see such a cross-over during the test-cross.
Tom Mueller - RHS
Morgan was the first to figure out that recombination could allow geneticists to map genes on chromosomes.
The Mugwump genes are 5 centiMorgans cM apart.(recombinantion happens with a 5% probability)
Gene mapping occurred long before DNA sequencing took place because of Morgan’s insight.
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Now on to some problems…
Tom Mueller - RHS